In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the unimaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. Developers are typically aqueous alkaline solutions, which may also contain substantial amounts of organic solvents. If the imaged regions are removed, the precursor is positive working. Conversely, if the unimaged regions are removed, the precursor is negative working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
The substrate typically comprises a support of aluminum or an aluminum alloy that has been treated to form a layer of aluminum oxide of its surface. The substrate may also comprise an interlayer on one or both surfaces of the substrate to modify the surface characteristics to enhance the hydrophilicity of the surface of the support, to improve adhesion to subsequent layers to the substrate, to protect the oxide from dye penetration of the imageable layer, and to protect the oxide layer during the development process. Typical interlayer materials are polyvinyl phosphonic acid (PVPA), vinyl phosphonic acid/acrylic acid (VPA/AA) co-polymers, and polyacrylic acid (PAA).
Current interlayer materials compatible with positive and negative-type imageable layers do not adequately protect the oxide layer of the support from high pH alkali developers. This causes decreased developer activity, increased developer replenishment, sludging in the processor and filter, and increased maintenance of the processor. Thus, a need exists for imageable elements that comprise an interlayer that protects the oxide layer of the support from high pH alkali developers, and can be used for a wide range of imageable layers.